Last weekend I had the absolutely amazing experience attending a Prehistoric Archaeometallurgy course at Butser Ancient Farm in the South Downs of Hampshire. Much like the Terramare Park I visited in Italy last month, at Butser they’ve been building reconstructions of prehistoric houses from all periods and experimenting with various prehistoric crafts, growing past crops and rearing ancient livestock for over forty years.
Butser is literally an archaeologist’s/child’s dream come true and I spent four days there under the expert tutelage of Dr. Simon Timberlake and Fergus Milton, designing and building a furnace and all the refractories (e.g. crucibles, tuyères, moulds etc.) in order to smelt ores to produce metal.
I’m not going to lie, I could write a blog post for every day I spent there, but that would be impractical I feel as I’d probably lose your interest halfway through Part Two of Four so here is a summary of highlights presented in ramblings, videos and pictures. Enjoy!
Side note: There is a whole load of specialised chemistry, techniques and practices involved in most of what I was doing – I’ve tried to avoid ladening this post with too much of that side of things, but if anyone wants to know more or discuss it further, please get in contact.
Following introductions, presentations, a tour of the farm and, of course, a tea break, we were split into groups of three and each group was going to be trying out different methods of smelting (basically the extraction of metal from the ore).
My group (consisting of me, a post-doc student called Aurélie and a guy called Richard) would be trying out a series of clay-lined bowl furnaces, which are pretty simple constructions. Dig a shallow hole in the ground, line it with clay, fill it with charcoal and set it on fire.
If you have a spare piece of garden it’s super easy to do it at home and it is now my dream to convert part of my future garden into a smelting/casting area (shhh nobody tell Robyn!)
The other groups were testing a bank furnace – dug into the side of a bank – and a post-hole furnace – dug deeper and narrower than a bowl furnace, like a post-hole.
First things first: we needed refractories and for that we needed clay. We were using locally sourced ground clay and tempering it with sand and…
Which we had to go collect ourselves.
Actually, it wasn’t as bad it sounds. The dung had dried and thus didn’t really have a smell anymore, but this meant it was necessary to crush it up before working it into the clay with the sand. Once we’d made a sufficient mix, we began shaping it into the various objects we needed i.e. crucibles, moulds and tuyères (tuyères are sections of hollow pipe about a foot long that are used to feed air from the bellows into the furnace), and then they were all set by an open fire pit to bake the clay at about 5-600°C and we left those overnight.
We then turned our attention for the rest of the day to our furnace pit, for which we did exactly as I described above – though admittedly we used modern tools to dig (no one tell the gods of prehistoric re-enactment!)
I’ll let Past Matt introduce Day 2:
So, the refractories survived (horray!) and it was time for bellow-making! For this we were really fortunate to have Dr. Brenda Craddock join us bearing stripped goat hides and guiding us through the stitching process.
It involved tag team of stitching work, using an awl and thread, making it air tight into a triangular sort of shape and strapping in wooden handles and a large hollow cane to generate and expel the air.
As you’ve probably noticed by now, there’s a lot of “probably”s and “hopefully”s involved in these various experimental activities, which is unfortunately just a condition of learning new skills – we’re inexperienced and there’s a lot of variables to juggle! The bellows were looking really good though and we were quick to hook them up to our tuyères and set them in our furnace. We were all ready and raring to go for the morning!
Days 3 and 4:
These were our smelting days and I could have not been more excited – this was also in part because we were doing an overnight iron smelt and I got to sleep in one of the roundhouses, which everyone should do – they’re really warm and comfortable! But as per usual, I digress…
We were going to be trying various methods of smelting to extract metal from the ore and because we were only there for another two days, it was decided we’d cram as many smelting scenarios into our furnaces at once, which although not the most scientific approach, it certainly gave us an idea of the potential variations in smelting methods in the past.
First we needed to crush the malachite (copper ore) though, which came in beautiful large chunks of green stone and require much hitting with a hammerstone to pulverise it into small pieces.
The cassiterite (tin ore) came ready powdered for us though. Over the course of a series of experiments we attempted to smelt (and co-smelt) copper from malachite ore, tin from cassiterite ore and bronze from a mixture of the two ores. We did this in several different ways:
- Layered freely within the charcoal
- In a crucible
- Crushed into small chunks
- Crushed to a fine powder, and
- Bound in a clay ball (think of a scotch egg scenario with the ore at the centre)
The advantage of combining these various techniques within a single furnace meant that we were able to readily compare how efficient they were at providing results.
Malachite will smelt to copper at about 7-800°C, whilst cassiterite requires temperatures of c.1000°C, so we needed to get our furnace up to that temperature before we put the ore in, and then sustain that temperature for anywhere between forty minutes and an hour, but at what point it would reduce to metal was total guesswork.
This required lots of air pumping and I’d wager that on Day Four we spent close to six hours on the floor, in the pounding sun, working the bellows.
Our success rate varied, though we did manage to produce little prills of all three types of metal.
The scotch-egg approach didn’t really work – we either burnt the ore or reduced it so far that it re-oxidised and became a shiny black mass. But the crucible and loose-layer approaches were really effective. If anything, our problem was working the furnace for too long, but that comes from experience I guess.
The refractories largely held up – what was interesting was that we’d added too much sand to our clay mix which had caused the end of the tuyères to turn to a glass-like substance, and much of the loose metal had adhered to the end, which made for a cool ornamental piece.
Heavily sunburnt, arms aching, choking on furnace fumes, soaked in sweat and most importantly incredibly happy, my time at Butser drew to a close with a series of mini-casting experiments testing our moulds.
I feel like this post barely scratches the surface of everything I experienced and learnt, but this trip really brought home the complexities of prehistoric metallurgy. I can’t imagine how Neolithic and Bronze Age societies developed and refined these processes to such an art. We produced only mere grams of our metals, but they were producing kilos of the stuff. If I could, I would do this kind of thing every day. Everything here really is awesome.
Acknowledgements: My extreme thanks go out to Fergus Milton, Dr. Simon Timberlake and Dr. Brenda Craddock, as well as the incredible team of staff and volunteers at Butser Ancient Farm, and of course the fantastic group of people that I got to work with over the four days.